Controlling Acetylene Adsorption and Reactions on Pt–Sn Catalytic Surfaces

Acetylene reactivity as a function of Sn concentration on Pt catalytic surfaces was studied by comparing adsorption and reactions of regular and deuterated acetylene at 90–1000 K on three surfaces, Pt(111), Pt₃Sn/Pt­(111), and Pt₂Sn/Pt­(111), using high-resolution electron energy loss spectroscopy, temperature-programmed desorption, and density functional theory calculations. The strongly adsorbed di-σ/π-bonded acetylene species, which dominate on pure Pt, were not detected on the Pt–Sn surfaces. The presence of Sn is also shown to suppress acetylene decomposition and, as a result, to maintain adsorbed acetylene in the molecular form as weakly adsorbed π- and di-σ-bonded species. The destabilization of adsorbed acetylene makes associative reactions with the formation of dimers (C₄ hydrocarbons) and trimers (benzene) progressively more energetically favorable with increasing Sn concentration. Acetylene adsorption modes and reactions on Pt catalytic surfaces can, therefore, be controlled with Sn alloying. The concentration of Sn needs to be an optimal level for catalytic activity since all hydrocarbon species bind preferentially only to Pt sites

Structure of Mo₂C_<i>x</i> and Mo₄C_<i>x</i> Molybdenum Carbide Nanoparticles and Their Anchoring Sites on ZSM‑5 Zeolites

Mo carbide nanoparticles supported on ZSM-5 zeolites are promising catalysts for methane dehydroaromatization. For this and other applications, it is important to identify the structure and anchoring sites of Mo carbide nanoparticles. In this work, structures of Mo₂C_<i>x</i> (<i>x</i> = 1, 2, 3, 4, and 6) and Mo₄C_<i>x</i> (<i>x</i> = 2, 4, 6, and 8) nanoparticles are identified using a genetic algorithm with density functional theory (DFT) calculations. The ZSM-5 anchoring sites are determined by evaluating infrared vibrational spectra for surface OH groups before and after Mo deposition. The spectroscopic results demonstrate that initial Mo oxide species preferentially anchors on framework Al sites and partially on Si sites on the external surface of the zeolite. In addition, Mo oxide deposition causes some dealumination, and a small fraction of Mo oxide species anchor on extraframework Al sites. Anchoring modes of Mo carbide nanoparticles are evaluated with DFT cluster calculations and with hybrid quantum mechanical and molecular mechanical (QM/MM) periodic structure calculations. Calculation results suggest that binding through two Mo atoms is energetically preferable for all Mo carbide nanoparticles on double Al-atom framework sites and external Si sites. On single Al-atom framework sites, the preferential binding mode depends on the particle composition. The calculations also suggest that Mo carbide nanoparticles with a C/Mo ratio greater than 1.5 are more stable on external Si sites and, thus, likely to migrate from zeolite pores onto the external surface of the zeolite. Therefore, in order to minimize such migration, the C/Mo ratio for zeolite-supported Mo carbide nanoparticles under hydrocarbon reaction conditions should be maintained below 1.5

Identification of Vertical and Horizontal Configurations for BPE Adsorption on Silver Surfaces

Adsorption of trans-1,2-bis­(4-pyridyl)­ethylene (BPE), a molecule with two pyridine rings connected with a CC double bond, was studied on Ag surfaces with surface-enhanced Raman spectroscopic (SERS) measurements and density functional theory (DFT) calculations. Spectroscopic measurements were collected using well-defined 48 nm monodispersed Ag and Au nanoparticles supported on SiO₂. Effects of Ag oxidation were evaluated by varying the duration of an ozone treatment prior to adsorption. Effects of surface coverage were evaluated by exposing unoxidized and oxidized Ag samples to solutions with a variable BPE concentration. Periodic unit-cell DFT calculations were performed using Ag(111), p(4 × 4)-O/Ag(111), and Ag₂O­(111) surfaces. Two adsorption configurations were identified: vertical and horizontal. In the vertical configuration, BPE adsorbs nearly orthogonal to the surface by binding through one of its N atoms to a single surface Ag atom. In the horizontal configuration, BPE adsorbs nearly parallel to the surface by binding through both of its N atoms to two separate surface Ag atoms. BPE adsorbs initially as a mixture of the vertical and horizontal configurations. As the BPE surface coverage increases, the vertical configuration becomes preferential due to geometric constraints. In contrast, the horizontal configuration becomes preferential with increasing extent of Ag oxidation due to its greater stability on oxidized surfaces. Similarities in spectroscopic results for metallic Ag and Au nanoparticles suggest that the BPE adsorption trends with increasing surface coverage are the same for both metals